1. Field of the Invention
The present invention relates to a piezoelectric drive type Micro Electro Mechanical System (MEMS) element which is used as a switch, a variable capacitor, or the like, and to a voltage control oscillator, a communication apparatus, and a method of manufacturing piezoelectric drive type MEMS element.
2. Description of the Related Art
In recent years, there has been progressive expansion in the range of application of MEMS technology in semiconductor processes. For example, in the field of high-frequency circuits, there are high expectations for the application of this technology to switches and variable capacitors.
An electrostatic drive mechanism employing static electricity has been used principally as the drive mechanism for a conventional MEMS switch or variable capacitor. This is because the material and structure are simple and the process is straightforward. A typical structure is one comprising a substrate provided with an electrode for generating electrostatic force and a fixed electrode for ohmic contact, and a conductive movable beam which is suspended over the substrate. By applying a voltage between the electrostatic force generating electrode and the movable beam, an attraction is created by the electrostatic force in such a manner that the fixed electrode and the movable electrode of the movable beam make contact with each other, whereby a switch is opened and closed. In order to achieve sufficient isolation of the switch, it is desirable that the fixed electrode of the substrate and the movable electrode of the movable beam should be set at least 1 μm apart. Since the electrostatic force decreases sharply in inverse proportion to the square of the distance between the electrodes, the spring constant is set to be relatively weak, and furthermore, in order to displace the movable beam by 1 μm or more, it is usually necessary to have a high voltage of 20 V or above as the electrostatic drive voltage. When the fixed electrode and the movable electrode make contact with each other, a strong force of attraction is generated, but since their separating action is dependent only on the force of the spring, then the separating force is weak and there is a possibility that the contact may become stuck and reliability is poor.
On the other hand, in recent years, developments have been made in piezoelectric film formation technologies for forming piezoelectric films directly onto a structural substrate, and investigation has also been carried out into piezoelectric drive mechanisms using piezoelectric force as a drive mechanism for a MEMS switch. A conceivable piezoelectric drive mechanism is one which comprises a substrate provided with a fixed electrode and a piezoelectric drive movable beam provided with a movable electrode suspended on the substrate and a piezoelectric element. The piezoelectric force is proportional to the voltage and is not dependent on the distance between the movable electrode and the fixed electrode. Furthermore, piezoelectric driving makes it possible to achieve greater displacement at a low voltage compared to electrostatic driving, and therefore the spring force can be raised accordingly, thus strengthening the separating force and making it possible to improve reliability.
Japanese Patent Application Publication No. 2005-313274 describes a piezoelectric drive type MEMS element, for example. Japanese Patent Application Publication No. 2005-313274 discloses an element having a structure comprising a movable part using a beam supported at both ends, as a piezoelectric drive type of MEMS element which can be manufactured stably and with good reliability.
However, the piezoelectric drive type of MEMS element described in Japanese Patent Application Publication No. 2005-313274 requires a complex process in order to manufacture the beam structure supported at both ends, involving: (i) providing a sacrificial layer on the substrate, (ii) forming a both-end-supported beam structure portion on the sacrificial layer, (iii) creating a cavity by etching from the rear surface of the substrate, and (iv) removing the sacrificial layer, and so on; hence, there are possibilities that it is difficult to raise the yield and lower costs.